This invention relates to a process for treating fly ash to render it highly usable as a concrete additive. The invention further relates to treated fly ash, to concrete mixtures containing treated fly ash, and to treated fly ash that entraps unwanted metals or heavy metals.
Fly ash is the finely divided mineral residue resulting from the combustion of pulverized coal in coal-fired power plants. As used herein, fly ash includes similar ashes produced by the combustion of other fuel materials, including but not limited to bark ash and bottom ash. Fly ash may also include a mixture of different ashes. Fly ash consists of inorganic, incombustible matter present in the coal or fuel that has been fused during combustion into a glassy, part amorphous and part crystalline structure.
Fly ash material is solidified while suspended in the exhaust gases and is collected by electrostatic precipitators or filter bags. Since the particles solidify while suspended in the exhaust gases, fly ash particles are generally spherical in shape and range in size from 0.5 μm to 100 μm. They consist mostly of silicon dioxide (SiO2), aluminum oxide (Al2O3) and iron oxide (Fe2O3), and are hence a suitable source of aluminum and silicon for geopolymers. They are also pozzolanic in nature and react with calcium hydroxide and alkali to form cementitious compounds.
Fly ash has been classified into two classes, F and C, based on the chemical composition of the fly ash. According to ASTM C 618, the chemical requirements to classify any fly ash are shown in Table 1.
TABLE 1Chemical Requirements for Fly Ash ClassificationFly Ash ClassPropertiesClass FClass CSilicon dioxide, aluminum oxide, iron oxide70.0 50.0 (SiO2 + Al2O3 + Fe2O3), min, %Sulfur trioxide (SO3), max, %5.05.0Moisture Content, max, %3.03.0Loss on ignition, max, %6.06.0
Class F fly ash is produced from burning anthracite and bituminous coals. This fly ash has siliceous or siliceous and aluminous material, which itself possesses little or no cementitious value, but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperature to form cementitious compounds. Class C fly ash is produced normally from lignite and sub-bituminous coals, and some class C fly ashes may contain significant amounts (higher than 10%) of calcium oxide (CaO) or lime. This class of fly ash, in addition to having pozzolanic properties, also has some cementitious properties (ASTM C 618-99).
Color is one of the important physical properties of fly ash in terms of estimating the lime content qualitatively. It is suggested that lighter color indicate the presence of high calcium oxide and darker colors suggest high organic content.
Coal combustion exhaust gases sometimes contain contaminants, such as heavy metals, that must be removed to meet environmental standards. This is often accomplished using activated carbon or other similar sorbents. The activated carbon is usually collected by electrostatic precipitators or filter bags together with the fly ash. Hence, the collected fly ash may be combined with carbon and adsorbed heavy metals. The carbon content may range up to 50% by weight, or more. Because bark ash has a high carbon content, fly ash that contains some bark ash may have a high carbon content.
While most fly ash is disposed in landfills or similar large waste containment facilities, increasing amounts of fly ash are used in the production of concrete. Fly ash may partially replace cement and improve several properties of concrete. However, not all fly ash is suitable for use as a concrete additive. For example, fly ash that contains carbon may absorb air entraining agents (AEAs), which are added to concrete in order to improve its workability and resistance toward freeze-thaw damage. When carbon adsorbs air-entraining agents, they become less available to entrain tiny air bubbles in the concrete which are required to lend the concrete its protection against freeze-thaw conditions. ASTM C 457 defines a standard test method for microscopical determinations of the air content of hardened concrete and of the specific surface, void frequency, spacing factor, and cement paste-air ratio of the air-void system in hardened concrete. ASTM C 457 may be used to determine how well the AEA is working. The degree carbon adsorbs AEAs is dependent on the surface area, type of carbon (very coarse particles or soot), and the polarity of the carbon. Activated carbon, the type commonly used to capture heavy metals and other contaminants in flue gases, effectively captures AEAs.
Air entraining agents can be costly. Fly ash is often added to concrete compositions because it is less expensive than the Portland cement it replaces. However, if the addition of fly ash to concrete compositions requires significantly increased amounts of AEAs, then there may be little or no cost savings gained by adding fly ash to the concrete composition. It would be an improvement in the art to provide a process for treating fly ash so that it substantially reduces the amount of AEA added to the concrete composition compared to untreated fly ash.
Concrete manufacturers and concrete users in the construction industry require concrete to have consistent, predictable properties. Fly ash carbon content can vary widely depending upon the power plant configuration, boiler type, coal type, etc. Differences in fly ash can affect the amount of AEA that must be added to produce the desired concrete properties. It would be an advancement in the art to provide a process for treating fly ash that substantially reduces the affect of varying fly ash carbon content. Such a process is provided herein.